1. Field of the Invention
This invention pertains to the field of video cameras, in particular, to color balance and exposure control circuits for a still video camera.
2. Description Relative to the Prior Art
In a still video camera, it is desirable that exposure time and optical aperture are both adjustable, thus varying, respectively, the length of exposure and the intensity of the light striking an image sensor in the camera. For example, in order to obtain a distinct still image of an object in motion, it is necessary to shorten the exposure time so as to prevent image blur. In shortening the exposure time it is ordinarily necessary to open the aperture so as to maintain the same total exposure. On the other hand, in order to obtain a distinct still image over a broad distance range, it is necessary to close down the aperture so as to extend the depth of field. Then the exposure time would have to be lengthened accordingly. A still video camera capable of such adjustments is shown, for example, in U.S. Pat. No. 4,541,016. The disclosed camera includes an aperture-preferred exposure control system in which the exposure time is controlled by comparing the peak level of the signal from the image sensor with a reference signal having an amplitude characteristic corresponding to the pre-set aperture.
In addition to controlling the exposure time, it is desirable to control the color balance of the camera. A video camera is correctly color-balanced when it reproduces a picture of a white card as a neutral white without any identifiable hue. (As a result, color balance for a video camera is often referred to as white balance.) Such a balance is ordinarily obtained when the card is illuminated with the light source used for recording and the red, green, and blue signal channels provide equal output amplitudes. If the light source is changed, such as in going from an indoor (tungsten or fluorescent lighting) to an outdoor setting, the camera must be rebalanced to provide the same output in relation to the white card with the new source of lighting (in this case, the sun). The usual practice is to hold the gain of the green channel fixed and alter the gain of the red and blue channels until the red and blue amplitudes match the green amplitude.
It is clearly inconvenient to have to manually rebalance the red and blue channels whenever illumination conditions change, and especially inconvenient to do so in connection with a white card. Moreover, the illumination should come from as wide an area as possible so that color measurements relate to the color characteristics of incident light on he subject, rather than to reflected light from the subject, which may possess an unusual color bias. The tendency, consequently, has been to find ways of predicting white balance by comparing illumination levels for isolated areas of the ambient spectrum (that is, without resort to a white card). In U.S. Pat. No. 4,584,598, and still video camera utilizes the luminous intensities of two areas of the ambient spectrum (mercury and certain infra-red wavelengths to identify the character of the light source and to provide corresponding gain adjustments.
These two patents illustrate that color balance and exposure control, which are distinctly separate procedures, ordinarily utilize distinct circuitry in functionally-different sections of the camera. Whereas exposure control is derived in relation to the image signal, necessitating (in the case of the '016 patent) special "in-line" processing of the image signal, color balance is preferably derived from a broader external view provided by dedicated sensors on the camera body. The color balance processing is accordingly done "off-line" in relation to the image signal.